It has long been recognized that chromatographic instruments are quite temperature sensitive. Retention time and peak heights of the components of the sample undergoing chromatographic analysis are a function of the temperature of the column. Thus when better resolution of component concentration measurements is required, it is desirable to provide chromatographs with temperature-controlling means to hold the equipment temperature within predefined limits.
Although numerous proposals have been offered for achieving this result, in general the separation column(s) have been placed within an enclosed chamber arranged in the form of an oven. The air in the chamber is temperature conditioned and usually elevated above ambient temperature and then forced to flow over the column to exhibit a uniform increased temperature along its entire length. Most often one or more thermostatically controlled electrical resistance heating elements are employed to control the temperature within the oven. Examples of such prior art heating and regulating systems are disclosed in U.S. Pat. Nos. 3,305,000 and 3,309,504.
These prior art systems are undesirable for use in certain applications due in part to their bulkiness which severely hampers their use as portable (i.e., man-carried) field-operable instruments. However, the disadvantages of the prior art are perhaps most accented because they involve active means to bring about the requisite temperature control, that is in the majority of applications electrical energy, in the form of current-generated electrical resistance heaters, is introduced. The required electrical power is not normally available with portable equipment because of its size, weight and service life requirements. In addition, the heaters, controllers, and other conventional equipment are not compatible with intrinsically safe equipment design criteria which is a desirable and often mandatory prerequisite for portable chromatographic equipment.